Let pipe B's speed be x
Hence the speed of pipe A is 2x
Now the speed of pipe C is 1/2(x+2x)
Thus the speed of pipe C is 3x/2
Together 3 pipes empty the pool in 6hrs 40 mins i.e. 400 minutes
If we take the ratio of all 3 pipes, C: B: A =3x/2: x: 2x
C: B: A=3: 2: 4
now time taken by A alone would be ((3+2+4)*400)/4
i.e equal to 900 minutes=15 hours
Hence Pipe A will take 15 hours to empty the swimming pool alone.